This has been an interesting thread. Motivated by Richard Lingren's
and John Denker's comments, I "wrote" a very simple Interactive
Physics module that looks at the motion of a uniform rod with a point
charge at one end initially at rest interacting with a uniform
electric field. While the verdict of Newton's second law is
clear--the CM should move along a STRAIGHT line in the direction of
the electric force with a constant acceleration--I suspect this may
not be intuitively obvious for many. It wasn't for me. (It is much
like the nonintuitively obvious result of laying a meter stick down
on the ground and kicking one end or that of hanging a magnetic
dipole from a string attached to one pole in a horizontally directed
magnetic field.)
Using IP it is relatively easy to look at the motion of the rod in
the center of mass frame and to observe that the motion is exactly
that of a pure dipole with half as much charge at EACH end in the
same uniform electric field. The rod simply oscillates back and
forth about its CM like a compass needle. None of this should be
surprising based on an analysis using Newton's laws, but it is,
again, somewhat nonintuitive.
So here's a small "paradox" that may be further enlightening to
resolve: In the accelerating frame, the torque is produced by the
combined action of electric and gravitational forces. But the
electric force acts at one END of the rod while the gravitational
force is distributed and can be considered (as usual) to act at the
CENTER of the rod. This being the case, why does the rod oscillate
about its center?